(ask) sitemap validator can not validate sitemap.xml

boostid

I have a client, we have sitemap.xml problem.
many site validator incl.google webmaster fetch can not access them,
many of them said it is run time out status or 404 code, while i can access normally if we type their sitemap.xml in address bar.
anyone can help me? is it server problem or others?
ps.we put sitemap.xml file on root server, so we can read it as normal url address, ([url]http://www.***domain.com/sitemap.xml[/url])